Answer : It depends on the definition of landscape architect. If you're asking if they design decks that are part of a larger landscaping scheme, then sometimes they do and sometimes they don't. If you're asking if they design standalone decks, then the answer is usually no.
Landscape architects typically focus on designing outdoor spaces that are in harmony with their surroundings, and decks are often seen as an extension of the home rather than as part of the landscaping. There are exceptions, of course, and there are some landscape architects who specialize in deck design, but for the most part this is not what they do.